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Hiya. I'm not sure it'll ever pay back just on gas savings, so you are probably right there. But it's important to place some value on having a secondary heat source, and some cooling, though that will be very rare.

At a COP of 3, expect to replace 4 units of gas (75% efficient), so 12.18p leccy v's 11.3p.

But, and here's when the numbers become more a leap of faith. Some of that leccy should be PV, I already had the PV, but I was exporting most of this. Today the ASHP went on at 8am and I've been importing little to nothing so far even though it's quite grey as total gen is around 900W.

Next, it's a dishonest comparison. I'm heating the living room (and sun room) with the ASHP, but saving on GCH for the whole house. The more the rest of the house cools, the less heat I lose as the differential between indoor and outdoor temps reduces.

I appreciate that the GCH will have to work harder when it comes back on, but I still save by having less 'all house' heat.

Also, whilst I'm happy to use a COP of 3, since the UK average is apparently 2.9 for an ASHP, do remember that in this case you will be avoiding night time use, and the worst of the winter use, so the average COP is probably better as you pick and choose the best times to use it.

Just to be clear, I'm not sure this is a great idea, it just seems like a good one to try, and a bit of fun, and as you mention, I'm pretty sure CO2 wise I'm doing the right thing as the PV I consume, or leccy I import, will cost the grid 2kWh's of gas (for every 1kWh of leccy), so taking the COP into account, should deliver 3-4kWh's of heat, giving a 2 to 1 benefit .... I think?

Mart,

Many thanks for the response.

I have never had a firm understanding of where and when I export (certainly not logged!). As the COP is greater than one, it is an absolute truth that in the shoulder months I would do better to stop diverting excess PV to the immersion, I should leave the excess to "run" the ASHP. I do have monthly logging of immersun diversion, that is all. So I could guesstimate that further away from "mid winter" I am going to be increasingly using PV over import.

The real winner that I had overlooked was that I should theoretically be able to switch off the whole house GCH a few weeks earlier than normal, and switch it back on again a little later. Whilst from a CO2 point of view I should leave it running all winter, arguing from a purely financial POV, the ASHP could be off for most of the winter where there wasn't the excess PV to run it.

Extra leccy import comes to 124kWh, and a total guess at diverting another 124kWh of PV leccy, makes it 248kWh of leccy, and about 500kWh of gas burnt for leccy generation.

I knew the theory here, but have to admit to being shocked.

Not sure if I am "the other guy" but just joined this forum following links from M.

But these are our first full year figures for the Air Conditioner/Heat Pump that was installed in November 2016.

As we recorded gas & electric consumption before & after we were able to compare year on year figures for Gas and Electric consumption.

Gas use had reduced by 5,588 kWh. That's an over 50% reduction compared to the previous year.

Electric import had increased by 344 kWh. That is an increase of 18% compared to previous year. But solar PV production had been lower in 2017 compared with 2016. Also, been using a lot of high power tools, cement mixer, etc in the garden last Autumn to extend the patio.

Use of Solar PV generated electric in 2017 had increased from about 40% to just over 48%.

So with our unit rates for electric & gas we saved £166.65 on gas, but spent £37.20 more on electric import. A total saving of £129.45 for the 1st year of having the airconditioner/heat pump.

Not sure if I am "the other guy" but just joined this forum following links from M.

Have a great big welcome. Nice to see you on board Savvy.

Yep, I've been singing your praises, and of course the success of the ASHP idea to back up GCH and make use of PV. As batteries go, for the bottom 6 months of the year, putting spare PV into thermal mass at a COP of 3+ seems like a perfect solution.

Not sure if I am "the other guy" but just joined this forum following links from M.

Have a great big welcome. Nice to see you on board Savvy.

Yep, I've been singing your praises, and of course the success of the ASHP idea to back up GCH and make use of PV. As batteries go, for the bottom 6 months of the year, putting spare PV into thermal mass at a COP of 3+ seems like a perfect solution.

Have fun on Navitron.

Mart.

Thanks for the welcome Mart. Interesting forum with lots of useful info.

So allowing for DHW and cooking we used approx 10% less than last year. The interesting bit then is when the weather is taken into account, and ASB (who is reasonably local to me) confirmed that this winter was colder than last, and advised that he used approx 1/3 more gas this year v's last.

So, still early days and assumptions based on weather, but working in kWh's, a rough guide might be +150 to 200kWh of leccy and -2,000 to 2,500kWh of gas. These are import figures, as I will of course have also consumed more PV generation, and reduced my export (with the house acting as a thermal store for the PV generation).

So seems like a success, and the living room / sun room door left open to direct the hot air into the living room is excellent for hanging soggy doggy towels on to dry out after hosing down a mud soaked Labrador now that almost every field is a partial bog.

I would say that's a resounding success. Here in Ireland I've noted an increase in gas consumption of about 35% this winter vs last on a large building I have the figures for.

Many thanks as I need some comparison data before jumping to any conclusions. I found a FT article suggesting a 1/3 more gas consumption this year than seasonal norms, but I suspect it meant during the 'beast from the east' not necessarily the whole 2017/18 period.

It was actually for a year to the end of March that it was up 35% in this case. Here at least it wasn't spectacularly cold at any time but it was noticeably cold pretty much all winter so the number of degree days was well above normal I'd imagine.